1. Field of the Invention
The present invention relates to a Raman amplifier amplifying a wavelength division multiplexed signal light by utilizing a Raman effect and an optical transmission system using the same and, in particular, relates to a technique for reducing the wavelength deviation of signal light power, which occurs due to the wavelength dependence of a Raman gain.
2. Description of the Related Art
Heretofore, in a conventional long-distance optical transmission system, an optical signal has been converted into an electric signal, to be transmitted by using a regenerating repeater, which performs the reshaping, retiming and regenerating. However, as an optical amplifier has been put into practical use at present, an optical amplification-repeating transmission system using the optical amplifier as a linear repeater is considered. It is expected that, by replacing the optical regenerating repeater with the optical amplification repeater, the number of parts in the repeater shall be substantially reduced and the reliability shall be ensured while the cost of the repeater shall be remarkably reduced.
Further, as one method for realizing the large capacity of an optical transmission system, attention is focused on a wavelength division multiplexing (WDM) optical transmission system for multiplexing two ore more optical signals of different wavelengths to transmit the multiplexed signals over one optical transmission path. In a WDM optical amplification-repeating transmission system that is a combination of the WDM optical transmission system and the optical amplification-repeating transmission system, it is possible to collectively amplify two or more optical signals of different wavelengths using an optical amplifier. Therefore, a large-capacity and long-distance transmission can be realized with a simple (economical) configuration.
For the repeater in the optical amplification and repeater transmission system, an erbium doped fiber amplifier (EDFA) is generally used. For example, a gain wavelength band of the EDFA is a 1.55 μm band (C-band) and a gain wavelength band of a GS-EDFA (Gain shifted-EDFA) shifted to a longer wavelength is a 1.58 μm band (L-band). Since each of the EDFA and the GS-EDFA described above has the gain wavelength bandwidth of 30 nm or more, it is also possible to use the two signal light wavelength bands together utilizing a multiplexer/demultiplexer corresponding to the C-band and the L-band, thereby realizing the bandwidth of 60 nm or more.
Further, an application of Raman amplification has been studied actively in recent years. When a silica-based fiber is used as an amplification medium, for example, a gain peak optical frequency of the Raman amplification becomes an optical frequency shifted to a lower frequency side by about 13.2 THz than a pumping light frequency supplied to the amplification medium. In other words, in terms of optical wavelengths, the Raman amplification has a gain band on a longer wavelength side than a pumping light wavelength and, for example, a Raman gain peak wavelength relative to the pumping light wavelength of 1.45 μm becomes near 1.55 μm shifted from the pumping light wavelength by about 100 nm.
In the following description, a difference between the pumping light frequency supplied to the amplification medium and the Raman gain peak frequency determined caused by the pumping light will be referred to as “Raman shift frequency”.
In an optical amplifier utilizing the Raman amplification described above, in order to obtain an amplification effect for a requested signal light wavelength, it is important that the pumping light wavelength can be set in consideration of the Raman shift frequency. Further, it has been known that a gain wavelength characteristic of the Raman amplification can be flattened, using a plurality of pumping lights having different center wavelengths. For example, it has been reported that a bandwidth of about 100 nm can be ensured as the gain wavelength bandwidth of the Raman amplification by adjusting oscillation wavelengths and the output optical powers of a plurality of pumping light sources (refer to Y Emori, et al., “100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodes”, OFC′99, PD19, 1999).
FIG. 24 is a diagram showing a configuration example of a conventional Raman amplifier.
In FIG. 24, the conventional Raman amplifier comprises: a multi-wavelengths pumping light source 101 generating a plurality of pumping lights P1-PK; and a multiplexer 102 supplying the pumping lights P1-PK output from the multi-wavelengths pumping light source 101 to an optical fiber 100 serving as an amplification medium, wherein the pumping lights P1-PK supplied from the multi-wavelengths pumping light source 101 to the optical fiber 100 via the multiplexer 102 are propagated in a direction opposite to that of signal lights S1-SL. In this conventional Raman amplifier, as shown in FIG. 25, for example, the wavelength allocation is applied, in which a signal light wavelength band wherein the plurality of signal lights S1-SL are arranged at required intervals, is separated from a pumping light wavelength band wherein the plurality of pumping lights P1-PK are arranged according to the signal light wavelength band. Further, there has also been known another configuration in which a gain equalizer 103 is used together with the above components, as shown in FIG. 26, in order to obtain a flat gain wavelength characteristic for the signal light wavelength band.
In order to realize a large-capacity and long-distance transmission system, it is important to extend (widen) the signal light wavelength bandwidth. In the case where the Raman amplification is utilized for realizing the wide bandwidth, it has been known that it is effective to apply the wavelength allocation, in which the signal light and pumping light are mixed in a certain wavelength band (refer to Japanese Unexamined Patent Publication No. 2002-229084, and Japanese Patent Application 2001-244798 and Japanese Patent Application 2001-390366, which are prior applications by the applicant of this invention).
FIG. 27 is a diagram showing a configuration example of the Raman amplifier to which the wavelength allocation described above is applied. Further, FIG. 28 is a diagram showing an exemplary wavelength allocation in the Raman amplifier of FIG. 27. In the case where some of the plurality of pumping lights P1-PM are mixed in the signal light wavelength band as shown in FIG. 28, an optical circulator 104 is used as an optical device for multiplexing the pumping lights as shown in FIG. 27. Here, the pumping lights that are not mixed in the signal light wavelength band are indicated as P1-PQ and the pumping lights that are mixed in the signal light wavelength band are indicated as PQ+1-PM. Further, the Raman amplifier may be configured similarly to that shown in FIG. 26 as described above, wherein the gain equalizer 103 (a broken line portion in FIG. 27) is used together, in order to obtain the flat gain wavelength characteristic over the entire signal light band.
In the Raman amplifier to which is applied the wavelength allocation described above wherein the signal light and pumping light are mixed in the certain wavelength band, it has been known that the Raman amplification occurs largely between the plurality of pumping lights and, therefore, the power required for the pumping light on the shorter wavelength side becomes extremely large. As a result, there is a problem in that a multiplexing composition of the pumping light source becomes complicated. Moreover, an amplified spontaneous emission light noise generated by the pumping light on the shorter wavelength side is increased and, therefore, a noise characteristic of the shorter wavelength side signal light is deteriorated.
In order to solve this problem, for example, there has been proposed a technique for improving a noise characteristic of the Raman amplifier and, at the same time, for reducing the pumping light power on the shorter wavelength side, by modulating the power of each pumping light and reducing the time-wise overlap of the power of the pumping lights each having a frequency difference close to the Raman shift frequency, to reduce the efficiency of the Raman amplification occurring between these pumping lights (refer to Japanese Patent Application 2002-334037, which is a prior application by the applicant of this invention.) More specifically, for example, each pumping light power is modulated as shown in FIG. 29. The example in FIG. 29 shows a change in each pumping light power for the case where the pumping lights of 8 wavelengths, arranged at wavelength spacing corresponding to ⅓ of the Raman shift frequency ΔfRAMAN (when a silica-based optical fiber is used as the amplification medium, ΔfRAMAN=13.2 THz, ΔfRAMAN/3=4.4 THz). Here, an optical pulse duty after the modulation is made to be 50%. The efficiency of the Raman amplification occurring between the pumping lights is increased to a maximal when the pumping lights are spaced from each other by the Raman shift frequency ΔfRAMM, namely in the example described above, when the pumping light P1 and the pumping light P4, the pumping light P2 and the pumping light P5, the pumping light P3 and the pumping light P6, the pumping light P4 and the pumping light P7, and the pumping light P5 and the pumping light P8 are combined with each other, respectively. Consequently, the power of each of the pumping lights is modulated so that the pumping lights are not overlapped in time-wise, to reduce the Raman amplification between each pumping light. Thus, it becomes possible to solve the problem described above.
However, the conventional Raman amplifier described above has problems as shown in (1)-(4) below:
(1) System Performance Degradation Due to an Increase of Loss at the Time of Gain Equalization
While a profile of the Raman gain relative to wavelengths, which is obtained by using pumping lights of a plurality of wavelengths has a plurality of peaks according to the number of wavelengths of the pumping lights as shown in FIG. 30, for example, the respective peak wavelengths show different values from gain peak wavelengths that would be obtained when the pumping lights are used independently. More specifically, the example of FIG. 30 shows a relationship between the Raman gain and the wavelength for when the pumping lights of two wavelengths, 1433 nm and 1464 nm, are used. In the case where the pumping lights are used independently, as shown by narrow curves in the figure, the peak wavelengths of the Raman gain profile are 1529.6 nm and 1565.5 nm, respectively. In contrast, in the case where the pumping lights of the two wavelengths are used concurrently as shown by a thick curve in the figure, the gain profile has peaks at 1538.2 nm and 1563.9 nm. The gain peak corresponding to the pumping light of 1433 nm is shifted to the longer wavelength side by 8.6 nm and the gain peak corresponding to the pumping light of 1464 nm is shifted to the shorter wavelength side by 1.6 nm.
Therefore, in the Raman amplifier using pumping lights of a plurality of wavelengths, even if the wavelengths of the pumping lights are allocated at equal spacing, the Raman gain profile obtained actually does not have gain peaks at equal spacing. As a result, when performing the gain equalization by using together a gain equalizer as shown in FIG. 26 and the like, it is necessary to use a gain equalizer having a complicated loss profile. However, such a gain equalizer generally has a large loss and may be a factor leading to system performance degradation. Further, for example, although there is an optical device realizing the complicated loss profile by itself, such as a known slant type fiber Bragg grating in which a grating direction is slanted relative to an optical fiber axis, a wavelength range within which the gain equalization can be performed in such a device is generally limited to about 40 nm or less. As a result, in a system having a wider signal light wavelength band, such as more than 40 nm, every time of the gain equalization, a series of processing of performing the gain equalization after demultiplexing signal lights, and then multiplexing the signal lights again is needed and, therefore, excessive losses in a demultiplexer and a multiplexer may become a problem.
(2) Inefficiency due to Arrangement of Pumping Light at Unequal Spacing In order to avoid the problem described in (1) above to enable the use of a gain equalizer with a simple configuration, for example, it is considered to select the wavelengths of pumping lights so that the peak wavelengths of the Raman gain profile are allocated at equal spacing. More specifically, for example, as shown by a solid curve in FIG. 31, in order to obtain four gain peaks at intervals of 23 nm, it is necessary to use the pumping lights of four wavelengths, 1430 nm, 1448 nm, 1470 nm and 1502 nm, the wavelength spacing therebetween are not fixed. If such pumping lights, the wavelengths of which are allocated at unequal spacing, are used, it becomes possible to use a gain equalizer with a simple configuration, having a periodic loss wavelength characteristic as shown by a dotted curve in FIG. 31.
However, as described above with reference to FIG. 29, in order to perform the Raman amplification between each pumping light efficiently in the wavelength allocation in which the signal light and the pumping light are mixed in the certain wavelength band, it is effective to set intervals of the pumping light frequency to 1/integer number of those of the Raman shift frequency and, the allocation at unequal spacing of the wavelengths of the plurality of pumping lights will result in inefficiency in the Raman amplifier to which the wavelength setting as described above is applied.
Further, in Japanese Patent Application 2001-244798 and Japanese Patent Application 2001-390366 described above, the applicant of the invention has shown that the system performance can be improved by using an optical filter narrowing the pumping light spectrum mixed in the signal light band or an optical filter rejecting a Rayleigh scattered light of the pumping light. At this time, if the pumping lights are arranged at equal spacing, it becomes possible to apply optical filters having periodicity for the respective purposes, and it becomes no longer necessary to use multiple optical filters. Therefore, advantages in terms of loss and cost are obtained. On the other hand, if the pumping lights are arranged at unequal spacing, the advantages as described above will be lost.
(3) System Performance Degradation Due to an Increase of Gain Deviation in the Signal Light Band on the Shorter Wavelength Side The Raman amplifier having an amplification bandwidth equivalent to or higher than the Raman shift frequency as shown in FIGS. 27 to 29 above has a characteristic in that the gain deviation is increased on the shorter wavelength side. This characteristic will be described specifically with reference to FIGS. 32 and 33. FIG. 32 shows an example in which a plurality of signal lights is arranged on the shorter and longer wavelength sides of a pumping light of a single wavelength (1460.3 nm, for example) and FIG. 33 shows a wavelength characteristic of the Raman gain obtained in the wavelength allocation of the signal lights and the pumping light of FIG. 32. As shown in each figure, the pumping light of 1460.3 nm mixed in the signal light band gives a gain to the signal lights on the longer wavelength (lower frequency) side than the pumping light, while giving a loss to the signal lights on the shorter wavelength (higher frequency) side than the pumping light. Further, although not shown in the figure here, it is known that an effective cross-sectional area of an optical fiber used as an amplification medium is reduced on the shorter wavelength side. Therefore, the Raman effect (loss) occurring between the pumping light and the signal lights on the shorter wavelength side than the pumping light has the higher efficiency than that of the Raman effect (gain) occurring between the pumping light and the signal lights on the longer wavelength side than the pumping light. In the specific example of FIG. 33, since an absolute value of loss peak is about 20.0 dB whereas an absolute value of gain peak is about 14.1 dB, it is understood that the Raman effect on the shorter wavelength side has the higher efficiency than that on the longer wavelength side.
Next, based on the wavelength characteristic of the Raman effect obtained by the pumping light of single wave as described above, the consideration is made on the gain wavelength characteristic of the Raman amplifier using pumping lights of a plurality of wavelengths. FIG. 34 shows an example of the gain wavelength characteristic calculated for the Raman amplifier having the amplification bandwidth about four times the Raman shift frequency. Note, the gain wavelength characteristic of FIG. 34 is a calculation result for the case where, in the Raman amplifier having the amplification bandwidth of 56.5 THz about four times the Raman shift frequency which is set to 13.2 THz, in other words, the amplification bandwidth of 405 nm corresponding to the range of from 1277.3 nm to 1682.3 nm, the pumping lights P1-P13 of thirteen wavelengths are arranged at equal frequency intervals of 4.4 THz (one third of the Raman shift frequency) (refer to Table 1 below), and also the Raman amplification between each pumping light is suppressed by modulating each pumping light as shown in FIG. 29 above.
TABLE 1PumpingOptical FrequencyOptical WavelengthLight(THz)(nm)P1244.91224.1P2240.51246.5P3236.11269.8P4231.71293.9P5227.31318.9P6222.91345.0P7218.51372.0P8214.11400.2P9209.71429.6P10205.31460.3P11200.91492.2P12196.51525.7P13192.11560.6
The Raman gain profile shown by a thick curve in FIG. 34 that is obtained when the pumping lights of thirteen wavelengths are used, is obtained approximately based on the overlap of each of the Raman gains that are obtained when the pumping lights P1-P13 shown by narrow curves in FIG. 34 are used independently. Further, here, the power of each pumping light is adjusted so that a minimal value of the gain by the total pumping lights becomes about 10 dB. In this calculation result, an average value of the gain by the total pumping lights is about 12.0 dB and the gain deviation is about 8.7 dB.
As shown in FIG. 34, since the pumping light contributory to the amplification near the gain peak of the longest wavelength only the pumping light P13 of the longest wavelength, a burden on the pumping light P13 at this gain peak is larger than at other gain peaks contributed by the plurality of pumping lights. Therefore, in order to obtain a flat gain, it is necessary to obtain a relatively large gain due to the Raman effect by the pumping light P13 of the longest wavelength. In this case, as apparent from the relationship shown in FIG. 33 above, the loss peak occurring on the shorter wavelength side than the pumping light P13 also becomes relatively large.
In order to compensate for this loss peak by the pumping light P13, the Raman effect by the pumping light P7 on the higher frequency (shorter wavelength) side than the pumping light P13 of the longest wavelength by 26.4 THz in terms of frequency needs to have a large gain. Similarly, the Raman effect by the pumping light P1 on the higher frequency side than the pumping light P7 by 26.4 THz in terms of frequency needs to have a further large gain. Therefore, the gain deviation on the shorter wavelength side will be especially increased. The data shown in Table 2 below summarizes maximal values and minimal values of the Raman gains corresponding to the pumping lights P1-P13 of respective wavelengths, and FIG. 35 is a plot of the data of Table 2, wherein the horizontal axis represents the wavelength and the vertical axis represents the gain.
TABLE 2MaximalMaximalMaximalMinimalMinimalMinimalPumpingWavelengthFrequencyValueWavelengthFrequencyValueLight(nm)(THz)(dB)(nm)(THz)(dB)P11301.2230.418.21310.3228.89.9P21322.4226.718.21336.0224.410.1P31348.6222.315.31362.7220.010.1P41378.4217.514.21392.4215.310.0P51408.1212.913.41422.2210.810.1P61438.5208.414.41450.4206.79.9P71466.0204.513.71479.7202.69.9P81495.2200.514.11513.3198.110.1P91528.8196.111.71547.7193.710.1P101565.5191.511.61584.5189.210.2P111601.5187.211.01620.5185.010.1P121640.9182.711.31659.1180.710.1P131672.9179.210.6
As apparent from Table 2 and FIG. 35, since the gain deviation on the shorter wavelength side is increased with the extension of the wavelength band of the signal light, the configuration of the requested gain equalizer becomes complicated and thus the loss is increased, and may be a factor leading to system performance degradation.
Next, a transmission loss wavelength characteristic of a typical single-mode optical fiber is shown in FIG. 38. A wavelength band of about 1.5 μm to 1.6 μm is a low loss wavelength band called C-band (1.530 μm to 1.565 μm) and L-band (1.565 μm to 1.625 μm). In a wavelength longer than about 1.6 μm, a transmission loss is abruptly increased due in large part to an infrared absorption loss, while in a wavelength shorter than about 1.5 μm, the transmission loss is increased due in large part to a Rayleigh scattering loss. Further, a wavelength in the vicinity of about 1.4 μm is a wavelength of OH-absorption loss caused by residual OH group possessed by the optical fiber, and therefore, the transmission loss becomes larger. Heretofore, there has been promoted the reduction of OH-absorption loss. For an optical fiber having a positive dispersion characteristic (positive dispersion fiber), there has been developed an optical fiber in which the OH-absorption loss is completely extinct in substantial. On the other hand, for an optical fiber having a negative dispersion characteristic (negative dispersion fiber), since a concentration of dopant, such as Ge is high, it is difficult to significantly reduce the OH-absorption loss. However, in view of the Raman amplification, it is advantageous to apply the negative dispersion fiber. This is because the negative dispersion fiber has a nonlinear effective core area smaller than that of the positive dispersion fiber, so that the Raman amplification can be effectively achieved. Accordingly, in a Raman amplifier to which the negative dispersion fiber is applied, there exists the OH-absorption loss possessed by the optical fiber, and therefore, there is caused a problem in that, if a pumping light wavelength is allocated in the vicinity of the absorption peak wavelength, the required pumping light power for obtaining a required gain becomes significantly larger.
Further, in the case where a pumping light wavelength, which is shorter by even number times a Raman shift frequency than the longest pumping light wavelength, is coincident with a wavelength of about 1.4 μm (about 214 THz), which is the wavelength of OH-absorption loss possessed by the optical fiber, the significantly large pumping light power is needed due to the transmission loss caused by a Raman effect (loss) and the OH-absorption loss.
As described above, in the Raman amplifier having a signal light wavelength bandwidth exceeding the Raman shift frequency, since a required gain of a specific pumping light wavelength becomes significantly larger due to the transmission loss caused by the Raman effect (loss) and the OH-absorption loss possessed by the optical fiber, there is caused a problem in that an optical noise characteristic is deteriorated due to an increase of pumping light power or a problem in that a gain deviation is increased with the extension of signal wavelength bandwidth, resulting in the deterioration of optical transmission system performance.
(4) System Performance Degradation Due to Complicated Shape of Gain Wavelength Characteristics
As can be guessed from the calculation result shown in FIG. 34 above, as the amplification bandwidth of the Raman amplifier is increased, the gain wavelength characteristic also become more complicated. For example, as shown in FIG. 36, in which an amplification band portion in the gain wavelength characteristic shown in FIG. 34 is enlarged, the gain wavelength characteristic in this portion has a complicated shape, which can be divided into a plurality of wavelength bands B1, B2 and B3 having respective gain deviations of substantially even magnitudes. Even if an optical filter having a periodic loss characteristic, which is typically used, is applied to the Raman amplifier having the gain wavelength characteristic of such a complicated shape, it is difficult to perform the efficient gain equalization.
FIG. 37 is a diagram showing an example in which the gain equalization is performed on the gain wavelength characteristic of FIG. 36 by applying the optical filter having a periodic loss characteristic. In this figure, a narrow curve represents a characteristic before the gain equalization, a thick curve represents a loss wavelength characteristic of the gain equalizer, and an extra thick curve represents a characteristic after the gain equalization. Here, two types of periodic optical filters are used to perform the gain equalization so that the gain deviation for the wavelength band B1 where the large gain deviation occurs in FIG. 36 can be reduced while ensuring a required gain. More specifically, two types of optical filters, one having a loss wavelength characteristic of a period of 3.76 THz, an amplitude of 3.0 dB and a center frequency of 230.4 THz and the other having a loss wavelength characteristic of a period of 4.41 THz, an amplitude of 1.2 dB and a center frequency of 231.1 THz, are used in combination as the gain equalizer. From the characteristic after the gain equalization represented by the extra thick curve in the figure, it can be understood that the gain deviation can be reduced and the gain around 10 dB can be secured for the wavelength band B1 on the shorter wavelength band, but the gain deviation is increased and the gain value is reduced significantly for each of other wavelength bands B2 and B3.
Therefore, in order to flatten the wavelength deviation of the Raman gain as shown in FIG. 36, it is required to apply a gain equalizer having the larger loss deviation and a more complicated loss wavelength characteristic. Such a gain equalizer has a large loss and therefore, may be a factor leading to system performance degradation. Further, even if the known optical device capable of realizing the complicated loss wavelength characteristic such as the slanted fiber Bragg grating, similarly in the problem described in (1) above, the wavelength range within which the gain equalization can be performed is limited. As a result, as the signal light bandwidth is increased, excessive losses are increased in the demultiplexer and the multiplexer, resulting in system performance degradation.